In physics you have "laws" -- unbreakable realities like the Law of Gravity and the Second Law of Thermodynamics.
Nothing is quite so simple in the world of wildlife where "rules" are generally observed but frequently broken. A few of the more interesting "rules"
- Allen's Rule states that certain extremities of animals are relatively shorter in the cooler parts of a species' range than in the warmer parts. By "extremities" we mean arms, legs, ears, and snout or nose. An example: the arms and legs of a Dinka tribesman from Sudan are much longer than those of an Innuit from Alaska.
- Bergmann's Rule: It's a basic physical reality that that the larger a solid object, the less surface area it has relative to its total volume. Therefore, large animals tend to lose heat more slowly, relative to their size, than smaller ones. Because of this fact animals tend to be larger in colder areas than in warmer ones -- an observation called "Bergmann's Rule". This is a very loose rule, but holds true for many species. For example, whitetail deer in Mexico are considerably smaller than their counterparts in Canada, while red fox and raccoon in Florida are generally smaller than their counterparts in Maine. What are we to make of tiny Eskimos and massive Samoans? Well, this is a rule not a law -- and rules are frequently broken. In fact, Bergmann's rule is really pretty weak. Eastern coyotes in North Carolina, for example, are much larger than their counterparts in Montana.
- Gloger's Rule states that dark pigments are more common in warm and humid habitats, while light-colored pigments are more common in cooler northern regions. An example of this is human skin coloration, but it is also true with bear (polar bears in Alaska and black bears in Alabama), rabbits and many other animal species. Again, however, this is a "rule" that is frequently broken.
- The Egg Rule states that among birds in the Northern Hemisphere, the average number of eggs in a clutch laid by songbirds (passerines) and raptors (hawks and eagles) increases as one moves north in latitude. This may occur because birds in northern zones have no hope of laying two clutches in a year.
Too much can be made of these "rules" as there is considerable variation within every animal population, not only between the sexes, but also in the genetic potential of each animal and its ability to achieve that genetic potential (i.e. the local availability of food). In the real world there is often as much variation between animals found in the same region as between groups of animals found in different regions, and often as much size variation between the sexes as there is within regions.